System Bits: April 24

Some superconductors carry spin currents
A few years ago, researchers from the University of Cambridge showed that it was possible to create electron pairs in which the spins are aligned: up-up or down-down. The spin current can be carried by up-up and down-down pairs moving in opposite directions with a net charge current of zero, and the ability to create such a pure spin super-current is an important step towards the team’s vision of creating a superconducting computing technology which could use massively less energy than the present silicon-based electronics. Now, the same team has identified a set of materials which encourage the pairing of spin-aligned electrons, so that a spin current flows more effectively in the superconducting state than in the non-superconducting (normal) state.

Conceptual image of spin current flow in a superconductor
Source: University of Cambridge

Professor Mark Blamire of Cambridge’s Department of Materials Science and Metallurgy, who led the research said, “Although some aspects of normal state spin electronics, or spintronics, are more efficient than standard semiconductor electronics, their large-scale application has been prevented because the large charge currents required to generate spin currents waste too much energy. A fully-superconducting method of generating and controlling spin currents offers a way to improve on this.”

In the current work, Blamire and his collaborators used a multi-layered stack of metal films in which each layer was only a few nanometres thick. They observed that when a microwave field was applied to the films, it caused the central magnetic layer to emit a spin current into the superconductor next to it.

“If we used only a superconductor, the spin current is blocked once the system is cooled below the temperature when it becomes a superconductor. The surprising result was that when we added a platinum layer to the superconductor, the spin current in the superconducting state was greater than in the normal state,” Blamire added.

Although the researchers have shown that certain superconductors can carry spin currents, so far these only occur over short distances. The next step for the research team is to understand how to increase the distance and how to control the spin currents.

Integrating optical components with existing chip designs
Two and a half years ago, a team of researchers led by groups at MIT, the University of California at Berkeley, and Boston University announced a milestone: the fabrication of a working microprocessor, built using only existing manufacturing processes, that integrated electronic and optical components on the same chip. But this approach required that the chip’s electrical components be built from the same layer of silicon as its optical components. That meant relying on an older chip technology in which the silicon layers for the electronics were thick enough for optics.

Now, the same team reported another breakthrough: a technique for assembling on-chip optics and electronic separately, which enables the use of more modern transistor technologies, and using existing manufacturing processes.

Researchers have developed a technique for assembling on-chip optics and electronic separately, which enables the use of more modern transistor technologies.

Source: MIT

Amir Atabaki, a research scientist at MIT’s Research Laboratory of Electronics and one of three first authors on the new paper said, “The most promising thing about this work is that you can optimize your photonics independently from your electronics. We have different silicon electronic technologies, and if we can just add photonics to them, it’d be a great capability for future communications and computing chips. For example, now we could imagine a microprocessor manufacturer or a GPU manufacturer like Intel or Nvidia saying, ‘This is very nice. We can now have photonic input and output for our microprocessor or GPU.’ And they don’t have to change much in their process to get the performance boost of on-chip optics.”

Moving from electrical communication to optical communication is attractive to chip manufacturers because it could significantly increase chips’ speed and reduce power consumption, an advantage that will grow in importance as chips’ transistor count continues to rise. In fact, the Semiconductor Industry Association has estimated that at current rates of increase, computers’ energy requirements will exceed the world’s total power output by 2040.

The integration of optical — or “photonic” — and electronic components on the same chip reduces power consumption still further. Optical communications devices are on the market today, but they consume too much power and generate too much heat to be integrated into an electronic chip such as a microprocessor, the researchers stressed.

A commercial modulator — the device that encodes digital information onto a light signal — consumes between 10 and 100 times as much power as the modulators built into the researchers’ new chip. It also takes up 10 to 20 times as much chip space. That’s because the integration of electronics and photonics on the same chip enables Atabaki and his colleagues to use a more space-efficient modulator design, based on a photonic device called a ring resonator.

“We have access to photonic architectures that you can’t normally use without integrated electronics,” Atabaki explains. “For example, today there is no commercial optical transceiver that uses optical resonators, because you need considerable electronics capability to control and stabilize that resonator.”

In addition to millions of transistors for executing computations, the researchers’ new chip includes all the components necessary for optical communication: modulators; waveguides, which steer light across the chip; resonators, which separate out different wavelengths of light, each of which can carry different data; and photodetectors, which translate incoming light signals back into electrical signals, the researchers explained.

Silicon — which is the basis of most modern computer chips — must be fabricated on top of a layer of glass to yield useful optical components. The difference between the refractive indices of the silicon and the glass — the degrees to which the materials bend light — is what confines light to the silicon optical components.

Using the manufacturing facilities at SUNY Polytechnic Institute’s Colleges of Nanoscale Sciences and Engineering, the researchers tried out a series of recipes for polysilicon deposition, varying the type of raw silicon used, processing temperatures and times, until they found one that offered a good tradeoff between electronic and optical properties. “I think we must have gone through more than 50 silicon wafers before finding a material that was just right,” Atabaki concluded.

Wearable smart tech
Thanks to a nanoscale transistor created by researchers at The University of Manchester and Shandong University in China flexible televisions, tablets and phones as well as ‘truly wearable’ smart tech are a step closer.

The international team has developed an ultrafast, nanoscale transistor – known as a thin film transistor, or TFT, – made out of an oxide semiconductor. The TFT is the first oxide-semiconductor based transistor that is capable of operating at a benchmark speed of 1 GHz. This could make the next generation electronic gadgets even faster, brighter and more flexible than ever before.

Wearable electronics requires flexibility and in many cases transparency, too. This would be the perfect application for our research. Plus, there is a trend in developing smart homes, smart hospitals and smart cities – in all of which oxide semiconductor TFTs will play a key role.”
Prof Aimin Song, Professor of Nanoelectronics

Aimin Song, Professor of Nanoelectronics in the School of Electrical & Electronic Engineering at The University of Manchester, explained. “TVs can already be made extremely thin and bright. Our work may help make TV more mechanically flexible and even cheaper to produce. But, perhaps even more importantly, our GHz transistors may enable medium or even high performance flexible electronic circuits, such as truly wearable electronics.”